1. Technical Field
This invention relates to a multi-beam exposure apparatus that performs exposure by imaging a multi-beam light source onto recording materials such as photoreceptors, light-sensitive materials and heat-sensitive materials.
2. Prior Art
Lithographic platemaking using PS (presensitized) plates is quite common in the printing industry. To print a color image, reading with a scanner is done in three separated colors R (red), G (green) and B (blue), the image signals for these three colors are converted to color separated halftone signals for four colors C (cyan), M (magenta), Y (yellow) and Bk (black), light-sensitive materials called xe2x80x9clith filmsxe2x80x9d are exposed for the respective colors by means of light beams modulated on the basis of the resulting color separated halftone signals so as to prepare lith plates for the respective colors, and halftone images for the respective colors are formed by exposure on PS plates using the prepared lith plates. As a result, lithographic printing plates of four colors C, M, Y and Bk are produced.
In recent years, direct platemaking and CTP (computer to plate) are drawing increasing attention since they contribute to simplifying the platlemaking process and shortening the time taken by it. These techniques eliminate the lith films and printing plates are made by drawing images directly on PS plates with light beams such as laser beams using the color separated halftone signals for four colors C, M, Y and Bk that have been acquired with the scanner system.
In order to produce print images of higher contrast and quality, the recording density mutst be increased up to 2400-2540 dpi so that the spot diameter of light beams that form halftone dots is reduced to about 10.0-10.6 xcexcm. While it is necessary to form finer beam spots by increasing the density of printed images, a further reduction of the platemaking time is required and PS plates as large as 1100 mmxc3x97950 mm are desirably exposed in the shortest possible time, say, within a period of several minutes. This requirement for accomplishing high-density exposure of large areas exists not only in the printing field but also in many image recording areas.
In the case of the above-mentioned large-sized PS plates, high-density exposure with a single light beam requires that the drum (external drum) fitted with the PS plate should rotate for main scan at a speed of 10,000 rpm or more. However, from structural and control viewpoints, this need is almost impossible to meet at low cost.
Since high-density exposure with a single light beam cannot be achieved in a shorter time, it has been proposed that the exposure time be shortened by drawing several lines with a plurality of light beams. An apparatus operating on this principle is called a multi-beam exposure apparatus and relevant prior art examples are disclosed in U.S. Pat. No. 5,517,359, Japanese Patent Application (JPA) No. 1864901994 and International Publication (WO) No. 97/27065.
U.S. Pat. No. 5,517,359 teaches an apparatus for imaging the light from a laser diode on a multi-channel linear light valve; the light from 19 emitters of a high-power (ca. 1 W) BALD (broad area laser diode) is imaged onto the linear light valve by means of a lens array in which the pitch between lenses is substantially the same as the pitch of the emitters; the images of the respective emitters are superposed and the small linear light valve is illuminated (coupled) with a high-power (20 W in total) LD (laser diode) array so that the desired image is formed on a heat-sensitive or light-sensitive material to realize effective CTP.
Since the small linear light valve array is illuminated with the 20 W high-power LD array, the apparatus requires fine adjustment of the relative positions of the two arrays. This poses two problems. First, if the LD light source fails, it must be replaced by a new LD array but the necessary adjustment is too complex to be performed by the user and the apparatus has to be brought to the factory or any appropriate service center where time-consuming repair and expensive parts replacement are performed. Second, in order to increase the reliability of the apparatus, the operating life of the high-power LD array has to be extended but this requires water cooling of the LD array, making the structure of the apparatus complex and increasing its cost.
The multi-beam recording apparatus disclosed in Japanese Patent Application (JPA) No. 186490/1994 comprises a plurality of light source portions each consisting of a discrete LD and collimating unit and which are arranged in a specified pattern to illuminate a perforated plate having a plurality of apertures formed in a pattern either identical or similar to the pattern of arrangement of the light source portions; light beams passing through the apertures are directed to imaging (reducing) optics so that they are imaged on a light-sensitive material (recording surface). With this recording apparatus, the individual light source portions need not be positioned in the specified pattern of arrangement in high precision and there is no need for prolonged adjustment but high-quality images can be recorded after simple adjustment.
If this apparatus is used to perform high-speed recording of large-sized PS plates, as many as several tens of light source portions must be used and in order to arrange them in a specified pattern, a light source unit of a comparatively large size must be employed.
The apparatus described in JPA No. 186490/1994 does not require as precise positioning as in the case where no perforated plate is used but, on the other hand, the apertures in the perforated plate must be aligned with the exit centers of the light beams from the respective LDs and replacement of a failing LD requires reasonably high positional precision and involves a complicated procedure. Second, due to the use of many expensive high-power LDs, the cost of the light source unit increases and the overall system reliability of the apparatus decreases. Thirdly, the light beams from all light source portions in the large-sized light source unit must be received by lenses, a parabolic mirror and other optical components of high precision and large size and, in addition, complex reducing (imaging) optics are required to reduce these light beams to a sufficiently small size on the recording surface of the light-sensitive material; these contribute to increasing the cost of the apparatus.
International Publication (WO) 97/27065 discloses an imaging apparatus for exposing platemaking materials and a platemaking apparatus using the same. In these apparatus, a plurality of 0.5-1.0 W optical fiber coupled LDs are arranged and a pattern of light beams emerging from the fibers are passed through telecentric optics so that they are imaged (exposed at smaller scale) on a platemaking material (heat-sensitive material or heat ablation material) fitted on an external drum so that the position and size of the exposing spot will have a specified precision in spite of changes in the distance from the exit end face of each fiber to the recording surface of the platemaking material.
If this apparatus is used in order to expose platemaking materials of the above-indicated large size within a duration on the order of several minutes, as many as several tens of LDs have to be used but then the cost of the apparatus increases and its overall system reliability decreases. If the number of LDs is reduced to, say, 24, the exposure time prolongs and the productivity decreases.
Ordinary laser printers use a polygonal mirror which deflects a single laser beam for main scan in a direction parallel to the rotating axis of a photoreceptor drum and they feature a much smaller size and a lower density than the platemaking apparatus. Japanese Utility Model Application (JMA) No. 137916/1986 proposes a laser printer which uses an acousto-optic light deflector (AOD) to deflect a laser beam in an auxiliary scanning direction (in which the photoreceptor drum rotates) so that a pluality of lines (raster) are recorded simultaneously by one cycle of main scan. In order to reduce the visibility of jaggies that frequently occur in low-density image forming apparatus, Japanese Patent No. 2783328 discloses an image forming apparatus that relies on the same principle of deflection and main scanning as the above-described laser printer and which uses an AOD or an electro-optic light defelctor (EOD) to perform deflection in a zigzag path so that odd and even lines are offset by half a pixel to ensure that oblique lines in characters and so forth will look smooth.
The above-described laser printer and image forming apparatus which use a polygonal mirror to deflect a laser beam for main scan have a common problem in that if multiple laser beams are used, the size of the polygonal mirror increases and controlling the polygonal mirror such that it rotates consistently becomes difficult to achieve or that if more than one polygonal mirror is used to handle the multiple laser beams, difficulty is encountered in controlling the polygonal mirrors. In any event, the polygonal mirror or mirrors are expensive and cannot be applied to the purpose of performing high-density exposure of large-sized platemaking materials.
The image forming apparatus disclosed in Japanese Patent No. 2783328 has another problem in that the pixel density cannot be adequately increased.
If a single light beam is used as by the apparatus disclosed in Japanese Utility Model Application (JMA) No. 137916/1986 and Japanese Patent No. 2783328, the method of recording two or more lines simultaneously during one cycle of deflection for main scan using an AOD, AOM or the like is not applicable to the purpose of performing high-density exposure of large-sized platemaking materials.
Turning back to the multi-beam exposure apparatus disclosed in U.S. Pat. No. 5,517,359, JPA No. 186490/1994 and WO 97/27065, if one wants to shorten the duration of high-density exposure of large-sized platemaking materials with a small number of multi-beams, the main scan speed has to be increased by increasing the number of revolutions of the external drum to, for example, about 2000 rpm or more. However, the drum capable of high speed rotation is not only very expensive but it also has the risk of causing the fitted printing plate to spin off. A lower speed of the drum is advantageous from the viewpoints of cost and safety but, on the other hand, the exposure time is prolonged.
If the number of multi-beams used in the multi-beam exposure apparatus is increased by a sufficient degree to achieve the intended high-density exposure of large-sized platemaking materials, the problems of high drum cost and prolonged exposure time are dissolved but, on the other hand, LDs or other light sources to be used for issuing light beams increases and the associated parts increase correspondingly in number to eventually increase the overall cost of the apparatus.
The increase in the number of light sources such as LDs causes the problem of higher failure rate. Suppose that if ten LDs are lit simultaneously, the first failure occurs 10,000 hours later. If a hundred LDs are lit simultaneously, the first failure occurs 1,000 hours later. This means that the shutdown period of the apparatus and, hence, the cost of servicemen increase. As a result, the reliability of the apparatus decreases.
An object, therefore, of the present invention is to provide a multi-beam exposure apparatus that is suitable for high-density recording on large-sized recording materials by multi-beam exposure and which is capable of exposing within a short time (1-3 minutes) without substantial increase in the number of light beams from light sources such as semiconductor lasers and without increasing the main scan speed such as the rotating speed of an external drum and which has the additional advantages of safety, a small number of parts, low cost, low failure rate of the light sources such as semiconductor lasers, high reliability of the exposing system, short shutdown time and low cost of servicemen.
This object of the invention can be attained by a multi-beam exposure apparatus comprising a light source for emitting a specified number of multi-beams spaced apart in the direction of auxiliary scanning, a deflecting unit for deflecting said specified number of multi-beams collectively on main scanning lines by a specified number of deflections such that the space between adjacent ones of said specified number of multi-beams is exposed, and a main scanning unit for performing main scan of a recording material as it is exposed with said specified number of multi-beams, further characterized in that the space between adjacent ones of said specified number of multi-beams is an integral multiple of (said specified number of deflections +1) multiplied by the pitch of pixels in the direction of auxiliary scanning.
In order to attain the object described above, the present invention provides a multi-beam exposure apparatus comprising: a light source for emitting a specified number of multi-beams spaced apart in a direction of auxiliary scanning; a deflecting unit for deflecting the specified number of multi-beams collectively on main scanning lines by a specified number of deflections such that a space between adjacent ones of the specified number of multi-beams is exposed; and a main scanning unit for performing main scan of a recording material as it is exposed with the specified number of multi-beams, wherein the space between adjacent ones of the specified number of multi-beams is an integral multiple of (the specified number of deflections +1) multiplied by a pitch of pixels in the direction of auxiliary scanning.
Preferably, the main scanning unit is a rotating outer drum having the recording material fitted on its peripheral surface.
Preferably, the light source is multi-beam emitting unit in array form.
Preferably, the light source is an optical fiber array emitting the multi-beams.
Preferably, the light source is an array of discrete semiconductor lasers emitting individual beams.
Preferably, the light source is a monolithic semiconductor laser array emitting the multi-beams.
It is preferable that the further comprises a collimator lens provided between the light source and the deflecting unit and an imaging lens provided between the deflecting unit and the recording material.
It is also preferable that the multi-beam exposure apparatus further comprises reducing optics provided in a plurality of stages between the deflecting unit and the collimator lens.
Preferably, the deflecting unit has an acousto-optic effect device.
Preferably, the acousto-optic effect device is an acousto-optic deflector.
Preferably, the acousto-optic effect device is an acousto-optic modulator.
Preferably, light of first-order diffraction and that of zeroth-order diffraction as being output from the acousto-optic modulator are adjusted to have equal intensity.
Preferably, the multi-beams are deflected by the acousto-optic effect device in a direction perpendicular to a direction in which the multi-beams are arranged.
Preferably, a direction of ultrasonic propagation from the acousto-optic effect device is adjusted to be perpendicular to a direction in which the multi-beams are arranged.
Preferably, the deflecting unit has an optical device capable of electro-optic effect.
Preferably, the multi-beams are deflected by the optical device capable of the electro-optic effect in a direction parallel to a direction in which the multi-beams are arranged.
Preferably, the multi-beams are deflected by the optical device capable of the electro-optic effect in a direction perpendicular to a direction in which the multi-beams are arranged.
Preferably, the deflecting unit comprises: a polarized beam splitting device for separating the multi-beams into two components according to a direction of polarization; a first polarization rotating device by which the direction of polarization of the component separated by the polarized beam splitting device is rotated so that the direction is parallel to the direction of polarization of the component that has passed through the polarized beam splitting device; a first and a second entity of the optical device capable of the electro-optic effect which respectively deflect a component that has passed through the polarized beam splitting device and a component that has been rotated in the direction of polarization by the first polarization rotating device; a second polarization rotating device for rotating the direction of polarization of a component that has been deflected by the first entity of the optical device capable of the electro-optic effect; and a wave coupling device by which a component of multi-beams that has been rotated in the direction of polarization by the second polarization rotating device is combined with a component that has been deflected by the second entity of the optical device capable of the electro-optic effect.
Preferably, the multi-beam emitting unit in array form are arranged in more than one arrow and the pixels that remain unrecorded between the multi-beams emitted from a single row of the multi-beam emitting unit are thoroughly recorded by the multi-beams emitted from all other rows of the multi-beam emitting unit.
Preferably, the pixels that remain unrecorded between the multi-beams emitted from the multi-beam emitting unit in array form are thoroughly recorded by interlaced exposure.
Preferably, the recording material is a photoreceptor, a light-sensitive material or a heat-sensitive material.